1. Field of the Invention
The present invention relates to a capacitor to be used in LSI, VLSI and ULSI chips or the like, which possesses a high dielectric constant and reduces a leakage current, and a method for producing the capacitor.
2. Description of the Background Art
A conventional dynamic RAM (dRAM), for example, includes a plurality of memory cells, each cell comprising a MOSFET and a capacitor, which are highly integrated to produce an LSI. In the LSI of this kind, a flat surface capacitor using a silicon dioxide (SiO.sub.2) film as a dielectric substance has been widely employed in the conventional capacitor. However, as the LSI has been more highly integrated, it has become difficult to cope with this high integration by only reducing the thickness of the SiO.sub.2 film of the capacitor, because the area of the capacitor is almost the same.
In order to solve this problem, silicon nitride (Si.sub.3 N.sub.4) film having a higher dielectric constant than that of the SiO.sub.2 film is used in combination with the SiO.sub.2 film in the form of a lamination structure such as a two layer laminate film of SiO.sub.2 film--Si.sub.3 N.sub.4 film, a three layer laminate film of SiO.sub.2 film--Si.sub.3 N.sub.4 film--SiO.sub.2 film and the like. However, the film thicknesses of these lamination films are approximately at most 5 nm in terms of the SiO.sub.2. In the very large scale integrated circuit (VLSI), the development of a dielectric substance having a higher dielectric constant has been demanded.
Now, research and development of a metal oxide of tantalum pentoxide (Ta.sub.2 O.sub.5) is most widely conducted for reason that the relative dielectric constant of Ta.sub.2 O.sub.5 is approximately 25 to 30 which is 6 to 8 times of that of the SiO.sub.2 or is 3 to 4 times of that of the Si.sub.3 N.sub.4. Hence, the thickness of the Ta.sub.2 O.sub.5 film can be thickened by the magnifying rate of the relative dielectric constant when the same capacitance as that of the SiO.sub.2 or Si.sub.3 N.sub.4 is to be obtained, to readily produce a film having less deficiency and to reduce an electric field when the same voltage is applied, resulting in reducing a load or charge given to an insulating film. Further, the tantalum (Ta) is one of materials whose refining techniques are most developed and high purity can be readily obtained. As to these high purity materials rather than the tantalum, for instance, titanium (Ti) itself has a high relative dielectric constant, but, since the oxide of Ti contains a metastable phase, a composition slip is apt to arise in the normal composition of TiO.sub.2, and the probability of the oxygen omission is high. Hence, the electric insulating property of TiO.sub.2 is inferior than that of Ta.sub.2 O.sub.5, and now the research and development of Ta.sub.2 O.sub.5 move on to practical use.
When the Ta.sub.2 O.sub.5 having such a high dielectric constant is formed on the silicon (Si) in the form of a thin film, its relative dielectric constant is reduced as the film is formed thinner. For instance, when the Ta.sub.2 O.sub.5 with a thickness of 20 nm is deposited, its effective relative dielectric constant is required to a low value such as 12 to 14.5 on account of a film of SiO.sub.2 or TaSi.sub.x O.sub.y having a low dielectric constant, which is produced at an interface therebetween. In particular, when the tantalum target is sputtered in a mixture gas atmosphere of argon (Ar) and oxygen (O) to deposit a Ta.sub.2 O.sub.5 film onto the silicon, the SiO.sub.2 is formed at the interface between the Si and the Ta.sub.2 O.sub.5 film. It is considered that the SiO.sub.2 is formed by the oxygen plasma in the plasma oxidation process. It is understood that the relative dielectric constant of the Ta.sub.2 O.sub.5 deposited on molybdenum (Mo) or platinum (Pt) does not depend on the thickness thereof. It is also understood that the Ta.sub.2 O.sub.5 has the same behavior on tungsten (W). Accordingly, when a metallic electrode is used, a high relative dielectric constant such as 25 to 30 can be obtained. However, when a metal film is deposited by a vapor deposition or sputtering, the higher the purity of the metal film, the more liable the metal film is formed in a needle crystal. For example, the leakage current of the sputtered Ta.sub.2 O.sub.5 film grown on the finely rough surface of the base metal film becomes large. It is considered that this problem is caused by the uneven thickness of the Ta.sub.2 O.sub.5 film and the electric field concentration effect due to the needle crystal.
In order to improve this problem, the roughness of the surface of the base electrode is reduced, as disclosed in Japanese Patent Laid-Open Specification No. 61-265856. This is performed by forming an amorphous structure by introducing oxygen or argon into the base electrode when or after the electrode is formed, or forming a quite thin thickness such as several nm of the base electrode, or forming relatively large particles in the base electrode by a heat treatment at a high temperature. Although these countermeasures achieve success to a certain extent, however, it is difficult to always obtain a stable reproducibility. Thus, a problem of reliability arises in a very large scale integrated circuit such as a 4M-dRAM, a 16M-dRAM or a larger scale dRAM. In order to ensure a higher reliability, the surface of the base electrode is to be positively smoothed and it is necessary to employ a structure which is capable of maintaining a chemical stability between a dielectric film and upper and lower electrodes.
In a conventional three-layer capacitor of a metal-high dielectric film-metal structure, the large leakage current is attributable to the uneven thickness of the high dielectric film due to the rough surface of the base electrode, and to the electric field concentration effect due to the uneven surface of the base electrode. Further, there exist silicides, nitrides, borides and carbides of metals, which do not react with the silicon semiconductor in a process at a high temperature of at least 650.degree. C. However, when a pure chemical compound contacts a metal oxide at a high temperature, the former takes the oxygen from the latter to deteriorate the insulation property of the metal oxide. In the conventional countermeasures for reducing the leakage current, no sufficient effects can be obtained so far in the LSI, the VLSI the ULSI or the like.